Along with popularization of the Internet and electronic mail, utilization of communication networks increases remarkably, so that an optical communication system allowing a large capacity of information transmission has been developed. There is an important meaning to improve a degree of wavelength division multiplexing in optical communication system more than that in the present situation. In this case, an optical wavelength multiplexing-demultiplexing element is considered one of the key devices. An optical wavelength multiplexing-demultiplexing element having an optical waveguide structure of an array grating type relates to a passive structure that has a narrow transmission width and a high extinction ratio. Furthermore, such an optical wavelength multiplexing-demultiplexing element has also a characteristic feature of capable of multiplexing and demultiplexing a number of optical signals.
FIG. 1 is a perspective view showing a constitution of a conventional arrayed waveguide grating wherein the arrayed waveguide grating 10 is prepared by defining channel waveguides for input use (hereinafter referred to as “inputting channel waveguides”) 12, channel waveguides for output use (hereinafter referred to as “outputting channel waveguides”) 13, a channel waveguide array 14 curved entirely in a U-shape, a first sector form slab waveguide 15 for connecting the inputting channel waveguides 12 with the channel waveguide array 14, and a second sector form slab waveguide 16 for connecting the outputting channel waveguides 13 with the channel waveguide array 14 on a substrate 11. The channel waveguide array 14 is constituted such that a length of each of the waveguides composing the channel waveguide array 14 becomes sequentially longer by a difference in waveguide length ΔL.
FIG. 2 is a schematic view showing a constitution of the first sector form slab waveguide on its input and output sides wherein a core of each waveguide in the inputting channel waveguides 12 has a tapered shape, which expands linearly in the vicinity of a boundary with respect to the first sector form slab waveguide 15, while a core of each waveguide in the channel waveguide array 14 has also a tapered shape, which expands linearly in the vicinity of a boundary with respect to the first sector form slab waveguide
When optical frequency characteristics are examined with respect to such a conventional arrayed waveguide grating 10, there is a parabolic loss characteristic in the vicinity of a central optical frequency in each of the waveguides. For this reason, when a wavelength of a laser light source deviates from its central optical frequency due to temperature change and the like, there has been a problem of increasing significantly such loss as described above. In order to prevent a sudden change in an output level due to such temperature change and the like as well as to prevent cutting in modulation components because of a narrowed transmission width in case of connecting arrayed waveguide gratings in a multiple stage, transmission characteristics having flat peak levels with respect to optical frequency are required.
Moreover, in a conventional optical communication system having a structure wherein plural stages of such arrayed waveguide gratings 10 are used for a communication network, loss characteristics (or light transmission characteristics) with respect to optical frequency in the case where a light ray passes through one of such arrayed waveguide gratings are repeated in the plural-stage arrayed waveguide gratings, so that loss characteristics with respect to optical frequency exhibit a more precipitous parabolic configuration in the vicinity of its central frequency. Thus, there have been such problems that such an optical communication system is easily affected in the case where a wavelength of a laser light source deviates from its central frequency and that modulation components are easily cut off. In this respect, although a variety of efforts have been made to improve parabolic loss characteristics in an array waveguide grating, it has been difficult to change such a present situation into a better one.